decode html encoded string back to original

mapping (the inverse of encode):

" " => " "
"&" => "&"
"&lt;" => "<"
"&gt;" => ">"
"&quot;" => "\""
"&#39;" => "'"
"&#x2F;" => "/"

Numeric entities of the form "&#<decimal>;" and "&#x<hex>;"
(e.g. "&#39;" or "&#x2F;") are supported. Illegal values
will result in `codepoint.replacement_character`

NYI: UNDER DEVELOPMENT: Named character references other than the once listed above are not supported (yet):
https://html.spec.whatwg.org/multipage/named-characters.html#named-character-references